I ask students which access point to the ocean Tucson is closer to, the Gulf of California or San Diego. When my students say that San Diego is closer to Tucson, there are several possible reasons for this, including that it must be closer because it is in the United States. This connects with other lessons in which we have discussed distance and scale. You can ask the same question, but base it on your location.

As the starfish in this lesson are from the Gulf of California, here is how I might ask the question if I lived somewhere else. "Which body of water is Boston closer to, the Atlantic or the Gulf of California," or the less obvious, "Which body of water is Boston closer to, the Pacific or the Gulf of California."

Ask students how many different species of sea stars live in the Gulf of California. If necessary, clarify for them the distinction between species (mountain lion and bobcat) versus a larger category (Felidae - the cats).

There is no expectation that they will have the answer to this! Record their guesses in order from least to greatest and, time permitting, show them the the mean, median and mode.

Ask them to give an explanation for their guess. Any answer is okay as long as they provide some justification. Here are a few examples of student guesses. Some children thought that a big area has more species. Most of the guesses in my class were numbers between 1 and 20, so when a child picked a larger number, I asked Billy justifies his estimate of 1000.

In the Gulf of California Marine Invertebrate Database and go to the search field. If appropriate for your group, you can talk about the hierarchy of the Linnaean system (keep in mind that the scientific community is now moving towards using clades, which rely on DNA analysis to determine interspecies relationships). The search for sea stars is conducted at the level of a class. (All mammals belong to the class Mammalia, all birds belong to the class Aves, and so on). Search for Asteroidea and let the students see that there are 64 different species of Asteroidea in the Gulf of California.

Option: If you would like to try out an alternative approach (teaching by mistake!) to emphasizing the importance of examining a data set before making a graph, look at the video clip in the Reflection. Here's a document that accompanies that clip: Searching for Sea Stars Graph with 1800 and more labels.

Tomorrow students will use information about the greatest and least depths (from either the database itself or a printed page, depending on the needs of your group and their access to computers) to round to the closest ten and create their own graph.

Big Idea:
In order to approximate a total, or to help check the reasonableness of an answer, students need to be able to round to the nearest 10 or 100. Check out this lesson for a fun game that gives them practice rounding, adding, and subtracting.